In a potential nanoscale breakthrough, scientists at Brown reveal 80-atom boron ‘buckyball’

PROVIDENCE R.I. [Brown University] - The nanoscale world appears to have a new ball to kick around.

Researchers from Brown University have shown the first experimental evidence for a "buckyball" molecule made from 80 boron atoms. The new structure is the cousin of the carbon buckyball, known formally as Buckminsterfullerene - a soccer ball-shaped molecule made from 60 carbon atoms that helped launch the nanotechnology revolution when it was discovered just over 40 years ago.

The evidence for the new nanostructure comes from photoelectron spectroscopy, which provides a sort of fingerprint for different molecular shapes and structures.

"I really didn't think this structure was going to be stable and that we were going to disprove its existence," said Lai-Sheng Wang, a professor of chemistry at Brown and the paper's corresponding author. "But when my student showed me the spectrum for the boron-80 cluster after I returned from a trip, I couldn't believe it."

The study is published in Chemical Science.

Carbon has long been the star of the nanotechnology world. The configuration of its electrons enables it to make all kinds of interesting shapes - including buckyballs, nanotubes and one-atom-thick graphene sheets - which have found uses in energy technology, medicine and more. Wang has been working for nearly 30 years to see if boron, carbon's neighbor on the periodic table, could make similar structures. If so, the structures could have even more interesting properties than their carbon cousins.

In 2013, Wang's team showed that clusters of 36 boron atoms formed a planar, one-atom-thick disc. By stitching those discs together, it would be possible to make borophene (a boron equivalent to graphene) - which two other labs indeed synthesized two years after Wang's discovery. In 2014, Wang's team showed that a 40-atom boron cluster formed a hollow cage similar to a buckyball, but lacking the perfect spherical symmetry.

For this latest research, Brown graduate students Hyun Wook Choi and Deniz Kahraman started by blasting a boron target with a high-powered laser. The impact knocks off a plume of boron atoms, which are then cooled quickly to form nanoclusters with various numbers of atoms. The clusters are then weighed to figure out how many atoms are present in each in a mass spectrometer.

To investigate the shapes of the clusters, the researchers use photoelectron spectroscopy. They zap each cluster with a second laser, which knocks an electron out of the structure and sends it flying down a long tube that Wang calls his "electron racetrack." The speed at which the electron flies down the racetrack is used to determine the cluster's electron binding energy spectrum - a readout of how tightly the cluster holds its electrons. That spectrum encodes information about the cluster's structure.

The photoelectron spectra of highly symmetrical structures have distinct peaks in their electron binding energy distribution. As Wang and his team investigated boron clusters larger than the 40-atom cage, the spectral readouts started to become relatively featureless, suggesting that the structures weren't particularly interesting. Wang said he started to doubt that the 80-atom cluster - which had previously been theorized to be a symmetrical ball - was actually going to be interesting.

"My attitude at that time was that this was probably going to be a low-symmetry thing," Wang said. "I thought, we'll get this ugly spectrum, publish it, then it's the end of the story for this B₈₀ cluster."

But the photoelectron spectrum for the cluster told a different story. The peaks in the spectrum stuck out like sore thumbs, which suggests a highly stable and highly symmetric structure. Working with colleagues in other labs around the world, Wang and his team determined that the only structure that could produce that surprisingly simple spectrum was the buckyball.

The findings are not without controversy. Density functional theory (DFT), the gold-standard method for determining molecular properties, suggests that the boron buckyball shouldn't be stable. Yet after an exhaustive search of possible 80-atom boron configurations, all signs pointed to the buckyball.

"I think the DFT calculations are wrong in this case," he said. "I think DFT has some of the bond lengths wrong for the B₈₀ buckyball, and that leads to incorrect predictions for its stability."

Wang says he hopes to work with colleagues at Brown and elsewhere to better understand why DFT might have gone awry in this case. In the meantime, Wang also hopes to work with colleagues in other labs to investigate the chemical reactivity of the B₈₀ buckyball that will be important to assess if boron buckyballs can be synthesized in bulk form. Wang's lab creates clusters in a vacuum, and it's not yet clear if boron buckyballs would be too reactive to stay intact in ambient conditions.

Just as with two-dimensional borophene, Wang is hopeful that the challenge of trying to make boron buckyballs in bulk can be overcome soon.

"It only took two years for borophene," Wang said, "so we'll see."